Serotonergic neurons of the medullary raphe magnus which project to the spinal cord have been implicated in the regulation of pain sensitivity. Activation of these neurons inhibits the transmission of nociceptive information in the dorsal horn of the spinal cord and results in analgesia. Although the ability of raphe magnus neurons to regulate the transmission of nociceptive information at the spinal level is well documented, little is known about what inputs regulate the activity of raphe magnus neurons themselves. Preliminary work in this laboratory has indicated that neurons of the raphe magnus are subject to modulation by both noradrenergic and cholinergic neurons. The proposed studies are designed to characterize the interaction among serotonergic, noradrenergic, and cholinergic neurons in the brain stem and spinal cord and to determine the functions of these neuronal systems in the regulation of nociceptive threshold. The proposed studies will involve: (1) microinjecting drugs which modify cholinergic and noradrenergic function in the raphe magnus and determining the resulting effects on pain threshold; (2) determining the effects of iontophoretically applied cholinergic and noradrenergic drugs on identified serotonergic neurons in the raphe magnus; (3) determining the capacity of spinally-projecting sertonergic and noradrenergic neurons to produce analgesia by activating them and modifying the resulting analgesia by spinal injections of drugs which alter monoaminergic function; and (4) determining whether the analgesia produced by electrical and pharmacological stimulation of brian stem neurons is mediated by the release of endogenous serotonin and norepinephrine. A detailed study of the pharmacology, anatomy and electrophysiology of these neuronal systems should provide important information about the mechanisms by which they regulate the transmission of nociceptive information. These pharmacological studies of pain modulatory systems may provide important basic information which could lead to the development of non-opiate analgesic drugs which do not possess the unfortunate side-effects of currently used optiates. Finally, the development in vivo spinal cord superfusion methods for directly measuring the release of monoaminergic neurotransmitters will provide important information onthe effects of drugs which alter the release of these transmitters.